The present invention relates to a process for producing a magnetic recording medium, and more particularly, to a process for producing a magnetic recording medium comprising a perpendicular magnetic film whose coercive force value is controllable over a wide range, which is excellent in oxidation resistance and corrosion resistance, and has a large squareness (value corrected as to demagnetizing field) and a high coercive force even when the cobalt content is as low as possible, in industrially and economically advantageous manner.
In recent years, there has been a remarkable tendency that information devices or systems are miniaturized and required to have a high reliability, and there has been an increasing demand for providing magnetic recording media capable of high-density recording. As magnetic recording media which can satisfy these requirements, there has been extensively developed perpendicular magnetic films. This is because such perpendicular magnetic films are free from demagnetization since these films are magnetized in the direction perpendicular to the surface thereof, thereby enabling high-density recording thereon.
Conventionally, as the perpendicular magnetic films, there have been proposed alloy films such as Co--Cr alloy film or the like. However, in order to prevent these alloy films from being deteriorated in magnetic properties due to the oxidation, it has been required to coat the surface thereof with a carbon film having a thickness of about 100 to about 200 .ANG., so that the loss due to the magnetic spacing (distance between a magnetic head and the magnetic recording layer) increases by the thickness of the carbon film, resulting in incompetence of high-density recording. For this reason, there has been a strongly demand for providing perpendicular magnetic films composed of oxides having a good oxidation stability.
Further, since it is necessary that the magnetic recording medium has a reproduced output as large as possible upon high-density recording, the perpendicular magnetic film used therefor is required to exhibit a squareness as large as possible. In addition, the coercive force of the perpendicular magnetic film is strongly required to be freely controllable over a wide range, especially 1,000 to 10,000 Oe.
More specifically, in order to carry out a magnetic saturation recording using current magnetic heads, the coercive force of the perpendicular magnetic film is strongly required to be about 1,000 to about 3,000 Oe. It is known that the coercive force Hc of magnetic recording medium and the writing ability of magnetic head have a close relationship to each other, and that when the coercive force Hc of the magnetic recording medium is too high, e.g., more than 3,000 Oe, the write current becomes higher, so that the current extensively used magnetic head fails to magnetize the magnetic recording medium to a sufficient extent because of a low saturation magnetic flux density Bm.
On the other hand, in order to provide a magnetic recording medium having a higher recording density, it has been strongly required that the coercive force value thereof is as high as possible, especially about 3,000 to about 10,000 Oe.
Conventionally, as the perpendicular magnetic films for magnetic recording, there have been proposed alloy films such as Co--Cr alloy film or Co--Pt alloy film, spinel-type iron oxide thin films such as cobalt-containing maghemite film (Japanese Patent Application Laid-Open (KOKAI) Nos. 51-119999(1976), 63-47359(1988), 3-17813(1991), 3-188604(1991), 4-10509(1992) and 5-12765(1993)), and magneto-plumbite-type iron oxide thin films such as barium ferrite film (Japanese Patent Application Laid-Open (KOKAI) No. 62-267949(1976)), or the like.
Among these perpendicular magnetic films, the cobalt-containing maghemite films which are most typical spinel-type iron oxide films, are excellent in oxidation resistance and corrosion resistance since maghemite is the iron oxide, resulting in excellent stability independent of change in passage of time and less change in magnetic properties with passage of time. Further, because of large crystal magnetic anisotropy, the cobalt-containing maghemite films are expected to be applied to perpendicular magnetic recording media.
The coercive force of the above-mentioned cobalt-containing maghemite thin films, becomes high with the increase of cobalt content thereof, while with the increase of cobalt content, the stability independent of change in passage of time thereof tends to be deteriorated by adverse influences of heat or the like. Therefore, it has been strongly required that the coercive force value is as high as possible even when the cobalt content is low.
Meanwhile, the cobalt-containing maghemite film has been produced on a substrate by a sputtering method or the like. The production process generally requires a temperature as high as not less than about 240.degree. C. As a result, there arises a problem that only aluminum discs, aluminum alloy discs or the like which can withstand a temperature as high as not less than 240.degree. C., are usable in the process. The conventional production processes in which the temperature as high as not less than 240.degree. C. is required, have such a problem that it becomes difficult to produce the cobalt-containing maghemite film in industrially and economically useful manner.
In recent years, in order to enhance a crystalline orientation property of the perpendicular magnetic films, it has been attempted to use a single-crystal substrate and to form various underlayers between the perpendicular magnetic film and the substrate. For example, there have been proposed a substrate composed of a MgO single crystal ("IEEE Trans. Mag.", MAG-12, No. 6, 733(1976), "IEEE Trans. Mag.", MAG-14, No. 5, 906(1978) and Czehch. "J. Phys.", B21, 563(1971)); a substrate composed of NaCl ("J. Cry. Growth", 50, 801(1980)); an underlayer composed of N o,(Japanese Patent Application Laid-Open (KOKAI) No. 5-166167(1993) and European Patent No. 0586142 A) or the like.
In the case of the sputtering method more frequently adopted among the above-mentioned conventional methods of producing a cobalt-containing maghemite thin film, there is caused such a disadvantage that although a magnetic easy axis of the cobalt-containing maghemite thin film is usually an axis &lt;100&gt;, the maghemite tends to be randomly oriented or the plane (111) thereof tends to be oriented in parallel with the substrate, so that it is difficult to produce a perpendicular magnetic film. As the method of producing a cobalt-containing maghemite film whose (400) plane is predominantly oriented in parallel with the substrate, there are known, for example, (a) a method described in "Lecture Abstracts of 9th Meeting of Japan Institute of Applied Magnetics", 29PB-10, (b) a method described in "Lecture Abstracts of 13th Meeting of Japan Institute of Applied Magnetics", p. 246, (c) a method described in Japanese Patent Application Laid-Open (KOKAI) No. 4-10509(1992), and (d) a method described in Japanese Patent Application Laid-Open (KOKAI) No. .sub.7 -307022(1995).
In the method (a), Fe and Co are ionized in an oxygen plasma and then deposited on an MgAl.sub.2 O.sub.4 substrate or a silica glass substrate which are heated to 500.degree. C. Thus, it is required to maintain the substrate at a temperature as high as not less than 500.degree. C. in vacuum during the film formation, resulting in poor productivity. In addition, since the substrate temperature is thus raised to not less than 500.degree. C., the substrate itself is required to show a good heat resistance. However, currently used materials for substrates of magnetic recording media such as glass, are insufficient in heat resistance at a high temperature of 500.degree. C. Accordingly, there are caused industrial and economical disadvantages that materials usable for the substrate are restricted.
The method (b) is a so-called plasma-excitation MO-CVD method in which the substrate is required to be maintained at a temperature as high as 300 to 400.degree. C. in vacuum during film formation process. For this reason, the method (b) is deteriorated in productivity and, therefore, industrially and economically disadvantageous.
In the method (c), after forming a multi-layered metal film composed of two or more Co layer(s) and Fe layer(s), the multi-layered metal film is heat-treated at not less than 500.degree. C. in an atmosphere containing oxygen. As described above, since it is necessary to conduct the method at such a high temperature, materials usable for the substrate are restricted, resulting in industrial and economical disadvantages.
In the method (d), a single-layered film composed of cobalt-containing magnetite whose (400) plane is predominantly oriented in parallel with the surface of substrate, or a multi-layered film composed of magnetite layer and CoO layer (in both films, in which the molar ratio of Co to Fe is not less than 0.01 and less than 0.10 is formed on an NiO underlayer which is formed on the substrate and whose (200) plane is predominantly oriented in parallel with the surface of the substrate, and then heat-treated at a temperature of 240 to 450.degree. C., thereby producing a cobalt-containing maghemite perpendicular magnetic film having a spacing of a plane (400) of not more than 2.082 .ANG.. However, since the production method still requires a temperature as high as not less than 240.degree. C., materials usable for the substrate are restricted, resulting in industrial and economical disadvantages. Further, in the method (d), the effect of obtaining a coercive force value as high as possible even when the cobalt content is low, cannot be achieved.
In the case where the afore-mentioned known substrate composed of MgO single crystal or NaCl is used, the plane (400) of the cobalt-containing maghemite thin film as a perpendicular magnetic film can be readily produced. However, the single-crystal substrate is not only expensive but also brittle, so that it is practically difficult to produce a large-area substrate therefrom.
From this point of view, the (200)-oriented NiO film can be practically used as an underlayer because the film can be readily produced on a glass substrate at room temperature by a sputtering method. However, in the case of the perpendicular magnetic film described in Japanese Patent Application Laid-Open (KOKAI) No. 5-166167(1993), although the crystal orientation of the plane (400) of a cobalt ferrite is accelerated by forming on the NiO underfilm, there arises such a problem that a compression stress is disadvantageously caused within the magnetic film because the spacing of plane (200): 2.09 .ANG. of the NiO underfilm is less than the spacing of plane (400): 2.10 .ANG. of Co.sub.x Fe.sub.3-x O.sub.4, thereby decreasing a perpendicular anisotropy.
This fact is apparently recognized from such a phenomenon that when the lattice constant of the underlayer is larger than that of the ferrite layer, the perpendicular magnetic anisotropy of the ferrite layer become larger, as described in Japanese Patent Application Laid-Open (KOKAI) No. 3-17813(1995).
Further, in European Patent Application Laid-Open No. 0586142, there is described a perpendicular magnetic film consisting essentially of a cobalt-containing maghemite thin film whose (400) plane is predominantly oriented in parallel with the surface of a substrate, and which is formed on an NiO underlayer whose (100) plane is predominantly oriented in parallel with the surface of the. The perpendicular magnetic film contains cobalt at a ratio of Co to Fe of 0.10:1 to 0.32:1, and has a spacing of plane (400) of not more than 2.085 .ANG. and a coercive force of not less than 4,000 Oe. In the production process, it is required to conduct a heat-treatment at a temperature as high as 280 to 450.degree. C., resulting in deteriorated productivity and, therefore, causing industrial and economical disadvantages.
It has been strongly demanded to provide a process for producing a perpendicular magnetic film which is excellent in oxidation resistance, and corrosion resistance and exhibits a large squareness (value corrected as to demagnetizing field) and a coercive force as high as possible even when the cobalt content is low, and whose coercive force is controllable over a wide range, in industrially and economically advantageous manner. However, there has not been provided yet such a method of producing the perpendicular magnetic film having all of the above-mentioned properties.
As a result of the present inventors' earnest studies, it has been found that by forming on the plastic substrate, the nickel oxide underlayer having a plane (200) which is predominantly oriented in parallel with the surface of the plastic substrate; forming on the nickel oxide underlayer, the cobalt-containing magnetite thin film having a plane (400) which is predominantly oriented in parallel with the surface of the plastic substrate, at a substrate temperature of not less 240.degree. C.; and oxidizing the obtained cobalt-containing magnetite thin film in a plasma-activated oxygen atmosphere containing a rare gas at a substrate temperature of less than 240.degree. C., a magnetic recording medium comprising a perpendicular magnetic film composed of spinel-type iron oxide, whose coercive force value is controllable over a wide range, which is excellent in oxidation resistance or corrosion resistance and has a large squareness (value corrected as to demagnetizing field) and a high coercive force even when the cobalt content is as low as possible, can be produced at a temperature as low as less than 240.degree. C. in industrially and economically advantageous manner. The present invention has been attained on the basis of the finding.